It's pretty simple
Good 3d cockpit and good upper surfaces - more resources into those views since thats all you see in game.
Well - then theres that external view of the airplane offline and in hanger, but I could see cutting corners on the lower surfaces, fuselage to stay within polycount if needed, but with a propererly set up LOD system, I don't think it should be a problem either way. You could probably even set up a higher poly wing/cockpit lod that would only be rendered from 1-3 yards out. Would allow for more detailed damage/parts falling off.